Apparatus and method for inerting a wave soldering installation

ABSTRACT

The present invention relates to an apparatus and a method for inerting a wave soldering installation having a solder bath ( 19 ) and a conveying system for producing one or more solder waves ( 14, 15 ), in particular for soldering electric printed circuit boards, having an immersion box ( 1 ) which is closed on all sides, shaped like a frame, can be immersed in the solder bath ( 19 ) and which has porous pipes ( 2, 3, 4 ) to distribute nitrogen, said pipes being arranged inside the immersion box in cage-like housings ( 5, 6, 7 ) with outlet openings ( 8, 9, 10 ), the cage-like housings ( 5, 6, 7 ) being designed such that the porous pipes ( 2, 3, 4 ) are arranged therein in such a way that the porous pipes ( 2, 3, 4 ) are essentially not struck by solder splashes produced during the operation of the wave soldering installation. By means of the arrangement and shape of the outlet openings ( 8, 9, 10 ), very uniform inerting of the wave soldering installation under all operating states can be achieved, even without an inerting covering hood. The elimination of solder splashes, which can damage the porous pipes ( 2, 3, 4 ), permits long service times; the construction of the installation permits simple maintenance.

[0001] The present invention relates to an apparatus and method forinerting a wave soldering installation having a solder bath and aconveying system for producing one or more solder waves, such as isused, in particular, for soldering electric printed circuit boards.

[0002] Wave soldering installations for soldering printed circuit boardsare known, for example from U.S. Pat. No. 5,121,874. In the case of thesystem described there, the inerting of the atmosphere over the solderbath is achieved by an elongated, closed covering, within which theprinted circuit boards to be soldered are transported and in which aninert atmosphere, generally nitrogen, is maintained. In the vicinity ofthe solder waves which are produced by the wave soldering installationduring operation, there are porous pipes which run parallel to the wavesand through which inert gas emerges, so that a particularly low-oxygenatmosphere can be achieved, in particular underneath the printed circuitboard to be soldered.

[0003] In addition, U.S. Pat. No. 5,411,200 and U.S. Pat. No. 5,409,159disclose the practice of surrounding distribution pipes for inert gasabove the solder bath of a wave soldering installation with cage-likehousings, which allow the inert gas to emerge through numerous outletopenings. In those documents, a description is also given of thepossibility of operating such a wave soldering installation with only ashort covering hood or even without a covering hood, by producing a verylow-oxygen atmosphere underneath the printed circuit boards by means ofskilful distribution of the inert gas above the solder bath during thepassage of printed circuit boards to be soldered, and even in the statewhen no printed circuit board is passing through, by producing aprotective layer of inert gas over the solder bath and the solder waves.

[0004] In addition, U.S. Pat. No. 5,361,969 also discloses the practiceof designing an apparatus for inerting a wave soldering installation asan immersion box which is closed at the sides and in which pipes fordistributing inert gas are mounted.

[0005] However, in the case of a wave soldering installation, threedifferent operating states may occur, specifically standstill of theinstallation (that is to say pumps switched off and, accordingly, solderwaves not present), installation operation without a printed circuitboard passing through and installation operation with a printed circuitboard passing through. For all three states, it is necessary to ensurethat little oxygen gets to the surface of the solder bath and the solderwaves. In addition, when a printed circuit board is passing through, itmust be ensured that a particularly inert atmosphere with only very lowproportions of oxygen of less than 10 ppm can be maintained underneaththis printed circuit board, where the soldering process is intended totake place, since otherwise the quality of the soldered joints becomespoorer. In addition, it is intended that such an installation should beoperated for as long as possible without maintenance and should be aseasy as possible to maintain. It is precisely on this point that theknown installations have weaknesses, since, for example, splashes ofsolder, which are always produced during the operation of a wavesoldering installation, can strike the distributor pipes for nitrogenand block these up over the course of time. This is critical inparticular when the distributor pipes used are porous pipes with veryfine pores.

[0006] The object of the present invention is therefore the provision ofan apparatus which permits the inerting of a wave soldering installationunder all operating conditions, with or even without a covering hood,and which is designed to require little maintenance and to be easy tomaintain. In addition, suitable methods for operating such an apparatusare to be specified.

[0007] In order to achieve this object, use is made of an apparatusaccording to claim 1 and methods according to claims 14 and 15.Advantageous and preferred refinements of the apparatus are specified inthe dependent claims.

[0008] An apparatus according to the invention for inerting a wavesoldering installation having a solder bath and a conveying system forproducing one or more solder waves, in particular for soldering electricprinted circuit boards, has an immersion box, which is closed on allsides, shaped like a frame, can be immersed in the solder bath and hasporous pipes to distribute nitrogen, the said pipes being arrangedinside the immersion box in cage-like housings with outlet openings, thecage-like housings being designed such that the porous pipes arearranged therein in such a way that the porous pipes can essentially notbe struck by solder splashes produced during the operation of the wavesoldering installation. According to the invention, the cage-likehousings around the porous pipes are used not only to distribute thenitrogen into the desired regions but in particular also to protect theporous pipes which effect a uniform distribution of the nitrogen withintheir housing. As distinct from the prior art, the cage-like housingsare configured not only from the point of view of the arrangement oftheir outlet openings but also from the point of view of the possibilityof preventing splashes of solder, entering through the outlet openings,getting to the porous pipes.

[0009] A particularly suitable arrangement for outlet openings hasproven to be an arrangement of outlet openings which are directed onlyto the sides and/or downwards. Since the most important objective of allthe arrangements for inerting has always been to achieve an atmospherewith the lowest possible oxygen content underneath the printed circuitboard during soldering, outlet openings were mostly also directedupwards against the circuit board. In addition, outlet openings werearranged in the inlet region and in the outlet region of thearrangement, and were intended to prevent the penetration of oxygen, inparticular in the case of installations without a large-area inertizedcovering. Surprisingly, however, outlet openings that are directed tothe sides and/or downwards are sufficient to ensure that the wavesoldering installation is inertized under all operating conditions. Ifno printed circuit board to be soldered is present, then in the case ofthis embodiment inert gas flows uniformly over the solder surface. If aprinted circuit board is present, then the inert gas can in any caseescape only in the inlet region and in the outlet region of the printedcircuit board, underneath the latter, so that the displacement of oxygenis in any case ensured there. The arrangement of outlet openingsaccording to the invention permits these to be arranged such that thereis no straight-line connection from the surface of the solder bath andthe solder waves through the outlet openings to the porous pipes, whichmeans that the probability that the porous pipes can be struck by soldersplashes is considerably reduced.

[0010] As an alternative or supplementary measure, the porous pipes canbe arranged decentrally inside their cage-like housings, being arrangedfurther from the respective outlet openings than from those walls of thecage-like housings which lie opposite the outlet openings. This alsoreduces the striking probability of solder splashes, without meaningdisadvantages for the distribution of the inert gas.

[0011] For any solder splashes which nevertheless penetrate through theoutlet openings, it is beneficial if the said splashes can run offdownwards into the solder bath through an intermediate space, within thecage-like housings, which is open at the bottom. For this purpose, it isalso particularly advantageous if all the walls of the cage-likehousings have an inclination with respect to the horizontal, so thatsolder splashes striking the walls on the inside can run off downwards,without striking the porous pipes. In particular, it is also impossiblefor any solder splashes to run along the walls above the porous pipe andthen to drop onto the pipes.

[0012] In particular for an arrangement without any covering which canbe inerted, it is beneficial if a first porous pipe is arranged on theinlet side of the apparatus, parallel to the solder wave, and a secondporous pipe is arranged on the opposite, outlet side.

[0013] If the wave soldering installation has two or more solder waves,then, according to the invention, a further porous pipe is preferablyarranged between each two solder waves and parallel to these, thecage-like housing of the said pipe being curved in a semicircular crosssection above the further porous pipe and being led down at the sides asfar as below the pipe, there being at least one outlet opening onlybelow the porous pipe. In this way, the porous pipe can virtually not bestruck at all by solder splashes, but permits inert gas to flow outprecisely where it is needed, specifically into the region of the flankof the solder wave. At those points where the solder wave flows backinto the solder bath, turbulence may be produced, which is aparticularly critical point for oxidation of the surface of the solderbath. Reliable inerting of this region is achieved by means of thearrangement according to the invention.

[0014] For solder waves which are located physically very close besideone another, the further porous pipe is selected to have a smaller crosssection than the other porous pipes, and is provided with a cage-likehousing that is located closely over it, so that there is a gap of 1 to3 mm between this housing and the porous pipe, through which gap theinert gas can flow uniformly. This arrangement is particularly compactand, in the case where the porous pipe and the housing are suspended onthe side walls of the immersion box, does not require any interferingbuilt-in fittings in the region in which the solder waves flow back intothe solder bath.

[0015] Use is preferably made, for the present invention, of porouspipes having a mean pore size of 0.3 to 2 μm, in particular 0.4 to 0.6μm. In general, porous pipes made of sintered metal are used, which havean expansion coefficient which is similar to that of the metallicimmersion box, so that fastening to the walls of the immersion box isrelatively simply possible. As a result of the reduced strikingprobability for solder splashes on the sintered metal pipes, accordingto the invention, it is also possible for very fine-pored pipes toremain in use without maintenance for a long time, and after that caneasily be replaced by being mounted simply in the immersion box.Fine-pored pipes permit a very uniform distribution of the inert gaswithin the cage-like housings, a very uniform flow of the inert gasthrough the outlet openings being produced, which is a precondition forgood inerting of the solder bath surface, even when there is no printedcircuit board present.

[0016] According to another aspect of the present invention, which isintended in particular to be suitable for wave soldering installationswithout an inerted covering hood, at least one guide plate isadditionally fastened to a cage-like housing in regions of turbulence inthe solder bath or in regions in which solder splashes are to beexpected to an increased extent, the said guide plate being able todeflect solder splashes downwards and to reduce turbulence as a solderwave is flowing back into the solder bath. It is particularly preferredfor there to be an additional outlet opening for inert gas underneathsuch a guide plate, so that the turbulent region underneath the guideplate is covered particularly well by inert gas.

[0017] As already mentioned, in the case of the present invention it isparticularly important to achieve good inerting even in operating statesin which there is no printed circuit board present above the wavesoldering installation. This is assisted if the outlet openings areconfigured so as to have a relatively large area and, as a result oftheir shape and arrangement, bring about a very uniform flow over thesurfaces of solder bath and solder wave.

[0018] It is particularly beneficial for the operation of the apparatusaccording to the invention if a greater flow of inert gas is led throughthe second porous pipe at the outlet side of the apparatus than througheach of the other porous pipes. Typically, in the case of a wavesoldering installation, the printed circuit boards are guided so thatthey rise slightly with respect to the horizontal, with the result that,in the outlet region, there is a greater distance between solder bathand printed circuit board or the upper edge of the immersion box than atthe inlet side. In addition, there is in this region, because of asolder wave that is present there, a region of increased turbulence,which is covered, for example, with a guide plate. An additional inertgas flow in this region is therefore useful in order to configure theinerting to be as beneficial as possible in this region as well.

[0019] It is particularly advantageous to supply the apparatus accordingto the invention with nitrogen at a temperature far below thetemperature of the solder bath, preferably 100 to 400K below thetemperature of the solder bath. In particular, nitrogen can be suppliedat an ambient temperature of about 20° C. This nitrogen initiallyoccupies a low volume as it is supplied and as it emerges from theoutlet openings, by which means low outflow velocities and hence a veryuniform to laminar flow are achieved. However, the nitrogen warms up inthe region of the immersion box above the solder bath, as a result ofwhich its volume becomes significantly greater. As a result, it fillsthe entire system very uniformly and escapes to the sides and upwardsand/or below the printed circuit board, at the sides of the latter, witha greater volume flow. This process may be somewhat less beneficial inenergetic terms than supplying preheated nitrogen or nitrogen at thetemperature of the solder bath, but leads to very good inerting. Sincethe porous pipes are not likely to be struck by solder splashes, it isalso not a disadvantage that these pipes are at a lower temperature, sothat solder splashes could stick to them.

[0020] An exemplary embodiment of the invention, but to which the latteris not restricted, is described in more detail below with reference tothe drawing, in which:

[0021]FIG. 1 shows a schematic longitudinal section through an apparatusaccording to the invention, and

[0022]FIG. 2 shows a view from above of the apparatus according to FIG.1.

[0023] The present exemplary embodiment relates to an overallarrangement of a wave soldering installation, such as is described, forexample, in the abovementioned U.S. Pat. Nos. 5,411,200 or 5,409,159, towhich reference is made fully here to this extent. The present inventionis concerned here only with the attachment to such an installation inorder to inert the surfaces. FIG. 1 shows a longitudinal section inschematic form through the apparatus according to the invention. Mountedin an immersion box 1, which is closed at the sides, are a first porouspipe 2, a second porous pipe 3 and a further porous pipe 4. These pipesare in each case arranged within cage-like housings 5, 6, 7, thesehousings having outlet openings 8, 9, 10. The porous pipes 2, 3, 4 arenot arranged centrally or symmetrically in the cage-like housings 5, 6,7; instead they have the greatest possible spacing from the respectiveoutlet opening 8, 9 or 10.

[0024] The entire immersion box is placed onto a wave solderinginstallation, part of the immersion box dipping into the solder bath 19,to be specific to such a depth that, even in the event of fluctuationsin the level in the solder bath, no gap can develop through whichambient air could penetrate. During the operation of the wave solderinginstallation, solder waves 14, 15 are located at the points indicatedschematically in FIG. 1. The outlet openings 8, 9, 10 and thearrangement of the porous pipes 2, 3, 4 in the cage-like housings 5, 6,7 are selected such that splashes of solder have only a very lowprobability of being able to strike the porous pipes 2, 3, 4. Inparticular, there is no straight connecting line from the solder bath 19or the solder waves 14, 15 through the outlet openings 8, 9, 10 to theporous pipes 2, 3, 4. Any solder splashes nevertheless penetratingthrough the outlet openings 8, 9, 10 can flow off towards the solderbath via intermediate spaces 11, 12, 13 which are open at the bottom,this being assisted by the fact that all the walls of the cage-likehousings 5, 6, 7 run at an angle, in particular at such an angle that nosolder can drop onto the porous pipes 2, 3, 4. This is also assisted bythe fact that the cage-like housings have no openings on the upper side.For reasons of space, it is often not possible to arrange, between twosolder waves 14, 15, a porous pipe with a cage-like housing whose wallsreach down into the solder bath 19. According to the present invention,therefore, a further porous pipe 4, preferably one having a smallercross section than the remaining porous pipes 2, 3, is arranged at sucha point, the said further porous pipe 4 being surrounded by a housing 7which is curved into a semicircle above the further porous pipe 4 and isled further down at the sides than the cross section of the porous pipe4. Produced in this way is at least one outlet opening 10, which isdirected downwards and at the same time, by means of an intermediatespace 13 that is open at the bottom, is so far removed from the furtherporous pipe 4 that solder splashes cannot reach as far as this pipe but,if necessary, can run off downwards. A gap 16 with a width of 1 to 3 mmis located between the further porous pipe 4 and its cage-like housing10. The porous pipes themselves are preferably produced from sinteredmetal and have a pore size between 0.3 and 2 μm, preferably 0.4 to 0.6μm.

[0025] In regions of increased turbulence in the solder bath 19, that isto say, in the present example, where the solder wave 15 flows back intothe solder bath 19 again, according to the invention a guide plate 17 ispreferably arranged on the cage-like housing 6 of the second porous pipe3. This guide plate 17 reduces turbulence and deflects solder splashesdownwards, at the same time leading inert gas, which emerges from anoutlet opening 18 underneath the guide plate 17, into the region of theturbulence and thus protecting the surface particularly effectivelyagainst oxidation at that point. The outlet openings 8, 9, 10 areconfigured in such a way that they produce a very uniform nitrogen flowin the direction of the interior of the immersion box 1 and/or along thesolder waves 14, 15. This is assisted if nitrogen at ambient temperatureor in any case at a temperature far below the temperature of the solderbath 19 is introduced as the inert gas. The volume of the nitrogen isthen still relatively low when it is introduced, which favours uniformflows without turbulence, the nitrogen then heating up above the solderbath and emerging upwards or to the side with a greater volume. As aresult, in the case where a printed circuit board is present, ambientoxygen is reliably kept away from the region of the printed circuitboard which is to be soldered. If there is no printed circuit boardabove the immersion box 1, the entire interior of the immersion boxnevertheless remains very largely inerted, specifically both during theoperation of the solder pumps and when they are stopped.

[0026] A further view from above of the immersion box is illustratedschematically in FIG. 2.

[0027] The present invention is suitable, in particular, for compactinstallations without an inerted covering, and enables the soldering ofprinted circuit boards with high quality with very long service times ofthe installation and simple maintenance.

1. Apparatus for inerting a wave soldering installation having a solderbath (19) and a conveying system for producing one or more solder waves(14, 15), in particular for soldering electric printed circuit boards,having an immersion box (1) which is closed on all sides, shaped like aframe, can be immersed in the solder bath (19) and which has porouspipes (2, 3, 4) to distribute nitrogen, said pipes being arranged insidethe immersion box in cage-like housings (5, 6, 7) with outlet openings(8, 9, 10), the cage-like housings (5, 6, 7) being designed such thatthe porous pipes (2, 3, 4) are arranged therein in such a way that theporous pipes (2, 3, 4) essentially cannot be struck by solder splashesproduced during the operation of the wave soldering installation. 2.Apparatus according to claim 1 , the cage-like housings (5, 6, 7) havingoutlet openings (8, 9, 10) which are directed only to the sides and/ordownwards.
 3. Apparatus according to claim 1 or 2 , the porous pipes (2,3, 4) being arranged decentrally inside their cage-like housings (5, 6,7), in particular further from the respective outlet openings (8, 9, 10)than from those walls of the cage-like housings (5, 6, 7) which lieopposite the outlet openings.
 4. Apparatus according to claim 1 or 2 ,there being in each case, between the outlet openings (8, 9, 10) and theporous pipes (2, 3, 4), an intermediate space (11, 12, 13) which is openat the bottom within the cage-like housings (5, 6, 7) and which permitssolder splashes to run off downwards.
 5. Apparatus according to one ofclaims 1 or 2, all the walls of the cage-like housings (5, 6, 7) havingan inclination with respect to the horizontal, so that solder splashesstriking the walls on the inside can run off downwards, without strikingthe porous pipes (2, 3, 4).
 6. Apparatus according to one of claims 1 or2, at least a first porous pipe (2) being arranged on an inlet side ofthe apparatus which lies parallel to the solder wave (14), and a secondporous pipe (3) being arranged on an opposite, outlet side, in each casein the upper region of the immersion box (1).
 7. Apparatus according toclaim 6 , in which, in the case of a wave soldering installation havingat least two solder waves (14, 15), a further porous pipe (4) isarranged between each two solder waves- (14, 15) and parallel to these,the cage-like housing (7) of the said pipe being curved in asemicircular cross section above the further porous pipe (4) and beingled down at the sides as far as below the further porous pipe (4), andhaving at least one outlet opening (10) only below the further porouspipe (4).
 8. Apparatus according to claim 7 , the further porous pipe(4) having a smaller cross section than the first (2) and second (3)porous pipes.
 9. Apparatus according to claim 7 , there being a gap (16)of 1 to 3 mm on the inside between the cage-like housing (7) and thefurther porous pipe (4).
 10. Apparatus according to one of claims 1 or2, the porous pipes (2, 3, 4) having a mean pore size of 0.3 to 2 μm, inparticular 0.4 to 0.6 μm.
 11. Apparatus according to one of claims 1 or2, in which at least one guide plate (17) is fastened to a cage-likehousing (6) in regions of turbulence in the solder bath and/or inregions in which solder splashes are to be expected to an increasedextent.
 12. Apparatus according to claim 11 , there being at least oneoutlet opening (18) for nitrogen underneath the guide plate (17). 13.Apparatus according to one of claims 1 or 2, all the outlet openings (8,9, 10) being configured such that the most uniform nitrogen flow withlow turbulence is produced.
 14. Method of operating an apparatusaccording to claim 6 , the flow of nitrogen through the second porouspipe (3) on the outlet side being greater than the respective flowthrough each of the remaining porous pipes (2, 4).
 15. Method ofoperating an apparatus according to claim 1 , the nitrogen beingsupplied at a temperature far below the temperature of the solder bath(19), preferably 100 to 400K below, in particular approximately at anambient temperature of about 20° C.